Coldest Eastern Rockies and plains climate of Colorado, Wyoming, Montana, and Southern Alberta
Centered over the plains of Wyoming and Montana, this zone sees January temperatures from 0 to 12°F (–18 to –11°C),with extremes between –30 and –50°F (–34 to –46°C). Zone 1b lies east of the Great Divide, where the continental climate reigns supreme. Arctic cold fronts sweep through 6 to 12 times a year, sometimes dropping temperatures by 30 or 40°F in 24 hours. The summer growing season tends to be warm and generous at 110 to 140 days long; but constant winds—12 miles per hour average, year-round in many places—call for windbreaks and shade trees, like hackberries and cottonwoods, whose leaves are animated by the wind.
Few shrubs are better loved here than lilacs, and few plants are better adapted than native ornamental grasses and prairie flowers. With protection, annual vegetables and flowers thrive, as do wind tolerant perennials such as buckwheats, grasses, and penstemons. Where hail is a problem, gardeners favor small-leafed plants; where winters are dry and snow cover light, they compensate with mulch and extra water.